Optical pointing devices are already known in the art. U.S. Pat. No. 5,288,993 for instance discloses a cursor pointing device utilizing a photodetector array and an illuminated target ball having randomly distributed speckles. U.S. Pat. No. 5,703,356 (related to the above-mentioned U.S. Pat. No. 5,288,993) further discloses (in reference to FIGS. 23A and 23B of this document) an optical cursor pointing device in the form of a mouse which does not require a ball and wherein light is reflected directly from the surface over which the pointing device is moved.
In both cases, the optical pointing device includes a light source for repetitively illuminating a surface portion (i.e. a surface portion of the ball or a portion of the surface over which the optical pointing device is moved) with radiation and an optical sensing unit comprising a photodetector array including a plurality of pixels each having a photosensitive element which is responsive to radiation reflected from the illuminated surface portion. The pixels outputs of the photodetector array are typically coupled to conditioning and processing circuits for tracking and extracting information about the relative motion between the sensing unit and the illuminated surface portion.
The technique used in above-cited U.S. Pat. Nos. 5,288,993 and 5,703,356 in order to extract motion-related information is based on a so-called “Edge Motion Detection” technique. This “Edge Motion Detection” technique essentially consists in a determination of the movement of edges (i.e. a difference between the intensity of pairs of pixels) in the image detected by the photodetector array. Edges are defined as spatial intensity differences between two pixels of the photodetector array. The relative motion of each of these edges is tracked and measured so as to determine an overall displacement measurement which is representative of the relative movement between the photodetector array and the illuminated portion of the surface.
An improved motion detection technique based on the above “Edge Motion Detection” technique is the subject matter of a pending international application No. PCT/EP 02/13686 filed on Dec. 3, 2002 (under priority of U.S. provisional application No. 60/335,792 of Dec. 5, 2001) in the name of EM Microelectronic-Marin SA and entitled “Method and sensing device for motion detection in an optical pointing device, such as an optical mouse” (published as International Application No. WO 03/049018 A1). The above international application describes various motion detection algorithms which are all based on a common basic assumption, i.e. that motion of the sensor with respect to the illuminated surface between two successive measurements is less than the pixel pitch, i.e. the spacing between adjacent pixels of the photodetector array.
Besides this algorithm assumption, non-linearity in the displacement detection curve is expected as shown in the graph of FIG. 1. This graph shows the motion detected by the sensor versus input (or “real”) motion, input motion being parallel to axis x in this example (input motion along axis y is zero). The input motion range is from 0 to 1 pixel pitch (1 in the graph of FIG. 1 means 1 pixel pitch). The targeted sensor output is shown by curve “a”, while curves “b” and “c” respectively show the sensor's outputs for motion along axis x and axis y.
FIG. 1 shows that as the input motion goes up (displacement speed increases) the detected motion and the output gain (i.e. the ratio between the detected motion and the input motion) go down. This kind of behaviour causes the gain (i.e. motion reported vs. real motion) to be a function of the sensor speed. The higher the speed, the larger the displacement between the sensor flashes and the lower the gain. This dependency is highly not desirable.
Another problem is that the gain curve shown in FIG. 1 changes for different surfaces. A different surface gain response is for example shown in FIG. 2, input motion being again parallel to axis x in this second example. There thus also exists a dependency of the gain on the surface, which is also not desirable.
A solution is thus required for compensating the non-linearity in the displacement detection curve of the above optical motion sensing device.